Black Carbon Absorption Efficiency Under Preindustrial and Present-Day Conditions Simulated by a Size- and Mixing-State-Resolved Global Aerosol Model

The positive radiative forcing of black carbon (BC) aerosol depends not only on the spatial and temporal distribution of BC but also its absorption efficiency. The mass absorption cross section (MAC) of BC is enhanced by atmospheric aging processes that increase particle size and non-BC coating amounts (mixing state) of BC-containing particles. However, the representation of MAC (or absorption enhancement) in current global aerosol models has a large uncertainty. This study used a global aerosol model that resolves particle size and mixing state to show that the MAC of anthropogenic BC has increased by 50% from preindustrial to present-day conditions (from 5.6 to 8.6 m(2) g(-1)) because faster present-day aging processes increase the fraction of thickly coated BC particles, which have high absorption efficiency. This effect is apparent only when the model considers BC mixing state with sufficient resolution. The impact of this MAC enhancement on BC direct radiative forcing is estimated to be 0.051-0.086 W m(-2)globally (22-37% of anthropogenic BC direct radiative forcing, 0.23 W m(-2)) and exceeds 0.5 W m(-2)near-source regions in East Asia. Sensitivity simulations show that BC direct radiative forcing and MAC are highly sensitive to non-BC emissions, secondary aerosol formation, and aerosol size distribution and mixing state in emissions. We must therefore improve our understanding of these factors by further observations and reduce their discrepancies between models to achieve better estimates of BC absorption efficiency and radiative forcing.